1. Field of the Invention
The present invention relates to a semiconductor wafer process, and more particularly, to a chemical mechanical polishing (CMP) method and apparatus for measuring an abrasion amount and friction force of a polishing pad using the thickness change of a slurry film in a CMP process.
2. Description of the Related Art
To manufacture a semiconductor device, various layers are deposited on a surface of a semiconductor wafer and patterned to form a circuit. In this stacking structure, wiring layers formed on different layers are connected to each other through vias and contact nodes. An intermediate layer is formed on the top surface of a lower wiring layer and planarized to expose the lower wiring layer. Then, an upper wiring layer is formed on the intermediate layer, thereby forming a stacked structure of wiring layers. A CMP process is generally performed to planarize the lower layer.
The CMP process is performed by a polisher. The polisher presses a contact surface of a polishing pad onto a contact surface of a semiconductor wafer to be polished. The polisher further supplies abrasive slurry in the space between the semiconductor wafer and the polishing pad and generates a mechanical motion of the polishing pad relative to the semiconductor wafer. The motion of the polishing pad relative to the semiconductor wafer can be generated using a belt type linear motion so that the polishing pad moves linearly, or a disk type rotational motion so that the relative motion is circular. In the CMP process, since the contact surface of the polishing pad is pressed on the contact surface of, and moves relative to the semiconductor wafer, a friction force is generated between the contact surfaces of the pad and the semiconductor wafer. The abrasive slurry supplied onto the contact surfaces thereby effectively removes the layer of the semiconductor wafer. The polishing pad should have a proper surface roughness such that a desired friction force can be generated as the polishing pad rubs the semiconductor wafer. The slurry should also be properly supplied into the space between the polishing pad and the semiconductor wafer. Since the surface condition of the polishing pad affects the polishing rate and polishing uniformity of the layer removed from the semiconductor wafer, the surface roughness should be maintained within an acceptable predetermined range.
To maintain the surface roughness of the polishing pad, a pad conditioning operation can be performed. In the pad conditioning operation, the contact surface of the polishing pad is continuously or intermittently mechanically rubbed either during the CMP process or after the CMP process. Unfortunately, although the pad conditioning operation helps maintain the surface roughness of the polishing pad in an acceptable range, the operation reduces the thickness of the polishing pad, and decreases the lifetime of the polishing pad. As the polishing pad is rubbed, the depths of grooves formed in the polishing pad decrease. When the depths of the grooves in the polishing pad are less than a predetermined value a proper amount of slurry cannot be supplied into the groove. Accordingly, under a predetermined pressure, the average thickness of a slurry film formed between the polishing pad and the semiconductor wafer changes. This thickness change of the slurry film reduces the pressure and the friction force between the contact surface of the polishing pad and the semiconductor wafer, thereby reducing the ability to remove the layer in the polishing operation.
Accordingly, since the polishing ability decreases due to the thickness decrease of the polishing pad, the polishing pad should be replaced when the thickness of the polishing pad is less than a predetermined acceptable value. In a conventional CMP process, the replacement time of the polishing pad is determined based on the accumulated use time of the polishing pad using a predetermined safety standard for managing the distribution of the CMP process. However, replacing the polishing pad based on the predetermined safety regulation does not provide an effective way of determining when to replace the polishing pad.
Various methods of optimally determining the replacement time of the polishing pad have been suggested. U.S. Pat. No. 5,934,974 discloses a non-contact method in which a laser sensor is used to directly measure a distance from the laser sensor to a surface of a polishing pad to calculate an abrasion amount of the polishing pad. In addition, U.S. Pat. No. 6,045,434 discloses a method in which an abrasion amount and abrasion profile of a polishing pad are monitored using ultrasonic or electromagnetic radiation transmitters arranged at a predetermined distance from the surface of the polishing pad to directly measure a distance to the surface of the polishing pad. However, in these methods, it is difficult to directly measure the distance between the semiconductor wafer and the polishing pad in a polishing operation because the polishing pad is always wet with water and slurry.
U.S. Pat. No. 5,743,784 discloses a method in which an abrasion state of the surface of a polishing pad is monitored in a conditioning operation, thereby determining an end point of the conditioning operation for the polishing pad. In this method, a disk type head is fixed on the surface of a polishing pad, and a friction force between the disk type head and the polishing pad is measured using a load cell. This determines the end point of the conditioning operation for the polishing pad. Unfortunately, however, the friction force between the disk type head and the polishing pad is significantly dependent on the conditioning degree of the polishing pad. Thus, this method is not suitable to monitor the abrasion or wear state of a polishing pad to determine its remaining useful life.